1. Field of the Invention
The present invention relates to an improved actuator. More particularly, the present invention relates to an actuator using a piezoelectric device. Further more particularly, the present invention relates to an actuator whose driving force and impact resistance are enhanced.
2. Description of the Related Art
Japanese Unexamined Patent Application No. 2002-95274 and Japanese Unexamined Patent Application No. 2004-15864 disclose an actuator that causes a moving member to be friction-engaged with a driving shaft that vibrates by a piezoelectric device and moves the moving member along the driving shaft by controlling the vibration waveform of the driving shaft. In the actuator of this type, the moving member is friction-engaged with the driving shaft such that a main body of the moving member engaged with the driving shaft and a spring clamp the driving shaft (via a holding member). Further, Japanese Unexamined Patent Application No. 10-80165 discloses an actuator that causes a driving shaft to be friction-engaged with a rotatable disk in the tangential direction in the vicinity of the disk, thereby being capable of controlling the rotation angle.
The conventional actuator described above gives acceleration, which exceeds the friction force by which the moving member is engaged with, to the driving shaft by the piezoelectric device for causing the moving member to slide relative to the driving shaft, wherein the driving force of the actuator is generated when the moving member is friction-engaged with the driving shaft and moved together therewith. Therefore, it is preferable that the spring is strengthened to increase the normal load to the friction face and the friction force by which the moving member is engaged with the driving shaft is increased. However, in the conventional actuator, the holding member pressed by the spring is in contact with the round surface of the driving shaft on the line in the shaft direction. Therefore, local concentration of the normal load may be generated due to a trivial factor such as a torsion between the driving shaft and the moving member or adherence of dust on the sliding face, so that there may be a case where great friction force is generated. The normal load is required to be set small to such a degree that the moving member can be slidably moved even in this case, thereby entailing a problem that the driving force cannot be set great.
Further, in a device having the actuator installed therein as disclosed in Japanese Unexamined Patent Application No. 2002-95274 and Japanese Unexamined Patent Application No. 2004-15864, acceleration of several thousand G is exerted on the moving member when it receives an impact due to, for example, a fall. The moving member may be fallen off over the spring or the spring may be damaged due to excessive force exerted thereon.